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    <meta charset="UTF-8">
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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<script>
    function strChange (str){
        var arr = str.split('')
       
        for(let i=0;i<arr.length;i++){
            if (arr[i]==arr[i].toUpperCase()) {
                arr[i]=arr[i].toLowerCase()
            }
            else{
                arr[i]=arr[i].toUpperCase()
            }
        }
        return arr.join('')
    }
    res = strChange('HHHDp')
    console.log(res);
</script>